1. Field of the Invention
Generally, the invention relates to the remote exchange of information between a portable electrical object, in particular an electronic memory card or chip card, and a terminal by modulation of the magnetic field generated by the terminal.
In this kind of remote data exchange, the magnetic field generated by the terminal serves to energize the electronic components of the portable object, and simultaneously to transmit digital data by modulation.
Various kinds of modulation can be envisaged.
2. Description of the Related Art
At present, use is generally made of frequency or phase modulation for transmitting information between the terminal and the object. In the direction of transmission from the portable object to the terminal, use is generally made of absorption modulation.
For transmission from the terminal to the portable object, it is also possible to envisage another type of modulation, namely amplitude modulation, this offering the advantage of avoiding the use of phase-locked loops and therefore leading to a simple system.
However, given that the portable object, energized by the magnetic field, is subjected to displacements within the magnetic field, especially when the user moves his card towards the terminal and then away from it, the voltage recovered at card level increases when the card is very close to the radiating antenna of the terminal, and may easily reach the breakdown voltage of the junctions of the transistors, thus leading to an impairment of the operation of the electronic components of this card. It is therefore necessary to limit this recovered voltage to a limit value. However, straightforward clipping of the voltage would cause the amplitude modulation of the latter to disappear, which modulation supports the information exchanges.